This invention relates to a photo-mask blank for use in lithography.
In general, a photo-mask blank of the type described is selectively etched or processed into a mask indispensable to the lithography technique of semiconductor integration. As a photo-mask blank, well-known is a chromium mask blank comprising a transparent glass substrate and a shading layer of chromium for shading incident light thereon.
In order to reduce the intensity of light reflected from the shading layer, the chromium mask blank very often comprises an antireflection layer of chromium oxide coated on the shading layer. The latter blank has a low reflectance, namely, a low reflection index and will, therefore, be referred to as a low reflection blank hereinafter.
In any event, the photo-mask blank is covered with a photo-resist layer on fabrication of the mask to delineate patterns on the photo-mask blank.
A conventional low reflection blank is manufactured by coating, in turn, the shading layer of chromium and the antireflection layer of chromium oxide on the transparent substrate by the use of sputtering and/or vacuum evaporation. As will later be described with reference to the figures of the accompanying drawing, the conventional low reflection blank exhibits a relatively high reflection index except a predetermined wavelength region when it is uncovered and covered with the photoresist layer. Therefore, the conventional low reflection blank inevitably imposes restrictions on the usable wavelength region.
Furthermore, the conventional low reflection blank is disadvantageous in that the shading layer of chromium is etched by an etchant faster than the antireflection layer of chromium oxide covered on the shading layer on fabrication of the mask. As a result, undercuts take place at the shading layer underlying an edge of the antireflection layer. This makes it difficult to delineate fine patterns on the glass substrate.